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charging 12V batteries in parallel

OK, so I'm rigging a parallel setup for charging three 12V batteries. Two full-size deep cycle and one smaller (motorcycle-sized but deep cycle) that's for my fish finder. I have a 1.25-amp charger. YES, I understand that it will take a great deal longer to charge with such a low-amp charger, but I will very rarely need to charge even two of them at the same time, much less three. This is mainly for ease of the maintain-charge.

If one depleted (say to 80% battery) is hooked up while the other two are at maintenance, what will the drawback be? Minor decreased battery life? I could unhook the at-charge ones to charge the depleted one and then parallel them all up for maintenance; is it worth it?

Re: charging 12V batteries in parallel

Charging dissimilar batteries in parallel is not a good idea at all. Charging similar batteries in parallel is better, but still not the best plan, unless your charger is designed for that use. Here's a boating forum with some more opinions:http://www.allboating.com/topics/003558.html?motors

Re: charging 12V batteries in parallel

Generally batteries to be charged in parallel should be similar in capacity, construction type ( eg. low maint, gel cell, flooded lead acid, starting vs/ deep discharge) and also should be at the same degree of discharge. Any degree of sulphation throws another considerable variable in to cause potential problems. What type of charger you have has some effect on how it electrically views combined batteries. The sooner a battery is recharged the better from sulphation perspectictive so a large capacity drained sidekick would interfere with the voltage coming up to what is needed to do anything for the smaller battery.

I hook a combined bunch of batteries I winter on in parallel to bring them up but the newer electronic " smart charger" then seems to have a different opinion on separate battery condition when hooked onto each individually.

Some people still think that letting a battery sit on a concrete floor will kill it, so you will find a hard time getting consistent advice. There is a fair bit of science that can be applied to it and lots of opinion but if you are into beekeeping you know about that, Lol!

Re: charging 12V batteries in parallel

if its for a boat why not just buy an onboard charger with however many banks needed.http://www.cabelas.com/product/Boati...3Bcat104465880
the problem with parallel is the old rule that electricity takes the path of least resistance meaning if the batteries are not identical the largest battery is going to probably absorb it all especially at that low amperage.

Re: charging 12V batteries in parallel

Any system seeks equilibrium. The charger doesn't have an unlimited capacity but typically maintains a maximum output voltage (about 13.5 volts) and current via electronics. The batteries on the other hand do what they do not unlike bees. When a charged battery is wired with a weak battery, electricity naturally flows from high to low. Once the actual (not rated) voltage becomes equal, no more flow, or flow only from the higher voltage charger. In the meantime, the batteries could be in for some fun. Diodes (electrical one way check valves) are one cheap way around this charging problem and keep batteries with the same voltage rating from exchanging power between them. Mixing voltage ratings (6 and 12 and 24 volts) is definitely a bad idea.

Re: charging 12V batteries in parallel

As throrope suggested, USE DIODES to keep your batteries from killing each other. Deep-cycle batteries are absolutely NOT "fast-cycle/cranking" batteries, they have more internal resistance, and can heat up & spill/take damage/explode if charged/discharged too quickly. So, hook a diode between your charging wire & the positive terminal of each battery individually to keep them from passing hazardous currents "back and forth."
Something I'm surprised nobody else has mentioned (directly, at least) yet: you'll want to put a small (0.01-0.1 ohm works great) resistor in series with the diode going to each battery, this helps "balance-out" the flow of electricity between the batteries, so one really big battery can't "hog all the juice" as easily.

Hope that helps (btw, I was studying for mech+elec. eng. before I ran out of tuition $$$ {SEVERAL years ago now}...so I'm not technically a "rocket scientist" but I do have a bit of experience in this field)

Re: charging 12V batteries in parallel

not to get too far off-topic here, but unfortunately I can no longer gain from going to school...not necessarily due to age, but due primarily to a newly "gained" learning disability....crippling short-term memory loss (most likely due to repeated combat-related head trauma, and exposure to explosions). Anywise, I do what I can...& I spend all of my free time (that's not demanded by my bees/sheep/dogs/ducks/crops) researching pretty much anything + everything. With any luck, I'll be able to make a positive contribution to "the sum of human knowledge" by applying the analytical reasoning skills that didn't get knocked out of me to whatever problems I can find to solve

Anywise, back to the topic at hand: I think you were dead-on with the diodes, and do you think the small voltage-drop across the diodes would be enough to allow for "leaving out" the balancing resistors in this circuit?